An OFDM modulation scheme can virtually convert frequency selective channel environments into multiple frequency non-selective channel environments, allowing the use of a multi-antenna technique without complicated equalization. Signal transmission using a multi-antenna scheme can improve the reliability in data transmission by achieving beamforming gain as well as diversity gain in fading environments.
It may be desirable for the base station (BS) to generate a beam weight based on channel information in order to maximize the performance of an OFDM based beamforming system. The BS generally use the channel information estimated by the mobile station (MS) because it cannot estimate the channel information of the MS in a Frequency Division Duplexing (FDD) environment.
As a consequence, a number of research works have been conducted on the feedback of channel information estimated by the MS to the BS, where the channel information is quantized using a codebook predefined in the BS and MS. Such a codebook-based channel information feedback scheme refers to a scheme for finding the best channel information from a codebook and reporting the index of the best channel information to the BS. Especially, a Grassmannian codebook scheme can efficiently generate a codebook with the use of less information by exploiting that the beam weight is generated indifferently from the phase. However, if the number of quantization bits (or bit size) used for the codebook is small, the quantization error increases, decreasing the beamforming gain. Since the channel information required for the beamforming increases in proportion to the number of subcarriers and symbols in an OFDM system, the signaling overhead for the feedback of channel information significantly increases.
To alleviate such shortcomings, a scheme for reducing the amount of channel information for the feedback by exploiting the correlation between contiguous symbols or subcarriers of OFDM has recently been proposed. The channel may have high correlation with adjacent channels in the presence of correlation between the sub-channels. Thus, the signaling overhead for the feedback of channel information can be reduced by re-defining adjacent symbols or subcarriers using a sub-codebook which has a smaller size than the original codebook. However, the quantization error induced by the original codebook cannot be reduced. Moreover, the improvement of the channel estimation accuracy is limited by using the channel correlation information in the time or frequency domain.